As well known, a shape memory alloy, such as a Ti—Ni alloy, exhibits a remarkable shape memory effect in association with martensitic reverse transformation. Further, the shape memory alloy exhibits excellent superelasticity in association with stress-induced martensitic transformation induced by strong deformation in a parent phase after the reverse transformation. The superelasticity is observed in a number of shape memory alloys and, among others, is particularly remarkable in the Ti—Ni alloy and a Ti—Ni—X alloy (X═V, Cr, Co, Nb, or the like).
The shape memory effect of the Ti—Ni alloy is disclosed in Patent Document 1. The superelasticity is disclosed in Patent Document 2. The shape memory effect and the superelasticity of the Ti—Ni—X alloy are described, for example, in Patent Documents 3 and 4 for a Ti—Ni—V alloy and in Patent Document 5 for a Ti—Ni—Nb alloy.
As compared with the Ti—Ni alloy, the Ti—Ni—Nb alloy exhibits a characteristic that temperature hysteresis of stresses is increased by applying a stress. Therefore, the Ti—Ni—Nb alloy is put into practical use as a joint for reactor piping.
Stent treatment is a new technique rapidly put into use in recent years. The stent is a mesh-like metal tube to be placed in a living body in order to prevent renarrowing or restenosis of a narrow portion, such as a blood vessel, after it is expanded. The stent is reduced in diameter and received in an end portion of a catheter. After introduced into the narrow portion, the stent is released from the catheter and expanded to be attached to an inner wall of a lumen such as a blood vessel. In case of PTCA (percutaneous transluminal coronary angioplasty), the stent is expanded following a blood vessel expanding operation by inflation of a balloon set on a housing inner wall. The stent is called a balloon expandable stent and formed by the use of a metal such as stainless steel or tantalum.
On the other hand, in order to prevent rupture of an aneurysm which may result in a subarachnoid hemorrhage or the like, blood supply to the aneurysm is stopped. As one of such techniques, use is made of embolization in which a metal coil, such as a platinum coil, is implanted into the aneurysm so as to form a blood clot. However, it is pointed out that a part of the blood clot may possibly be released from the metal and carried by a bloodstream to a periphery to block a blood vessel. In order to avoid this, consideration is made about a covered stent technique in which the aneurysm is embolized by the use of a graft. In this case, simultaneously when the stent is released from the catheter, the stent is expanded by its own spring function to press the graft against a blood vessel wall. Such stent is called a self expandable stent. For the self expandable stent, a material having an excellent spring characteristic is desired.
The superelastic Ti—Ni alloy is characterized in that, at a temperature above a reverse transformation finish temperature (Af point) at which reverse transformation of the alloy starting from a reverse transformation start temperature (As point) is finished, the alloy which has been deformed under an external load is recovered into an original shape simultaneously when the external load is released and that recoverable deformation reaches about 7% in case of an elongation strain. The As point means a shape recovery start temperature while the Af point means a shape recovery finish temperature (shape recovery temperature). For use as the stent, a hoop-shaped stent is formed into a size slightly greater than the lumen where the stent is to be placed. The stent is reduced in diameter and mounted to the catheter. Simultaneously when the stent is released from the catheter, the stent is spontaneously recovered into its original hoop diameter to be brought into tight contact with the lumen such as a blood vessel. Thus, the alloy has the Af point not lower than a living body temperature and always exhibits superelasticity at the living body temperature (around 37° C.).
As well as the above-mentioned merits, such superelastic stent has several demerits, such as occurrence of damage in the blood vessel wall, a positioning error in placement, lack in deliverability, and so on due to its spontaneous shape recovery characteristic. Therefore, it is difficult to use the superelastic stent in a blood vessel system such as a coronary system.
An example of a shape memory alloy used as a temperature-sensitive actuating device is described in Non-Patent Document 1 or the like. There are many examples, such as wind direction adjustment of an air conditioner, a damper of a microwave oven, and a ventilating hole. However, most of the examples utilize an R phase obtained by aging or thermomechanical treatment of the Ti—Ni alloy. The reverse transformation start temperature is about 60° C. On the other hand, an attempt for a high-temperature operating device to which this invention is applicable is described in Non-Patent Document 2. In this document, an operating temperature not lower than 100° C. is obtained by applying high strain to the Ti—Ni alloy. However, there are problems that the applied strain is large and that the operating sensitivity is low. Therefore, practical application is not yet realized.
The stent for PTCA is preferably made of a metal material having a low elastic limit, which hardly damages the blood vessel and is excellent in deliverability. However, there is left a problem that a pressing force (expanding force) against a lumen wall after expansion is weak. As means to solve the problem, a stent using a shape memory alloy is proposed. Patent Document 6 describes that a Ti—Ni—Nb alloy, to which this invention is related, is applied to a stent. Patent Document 6 describes that the stent made of a Ti—Ni—Nb shape memory alloy and having a low Young's modulus upon shape recovery and a high Young's modulus upon shape deformation under an external load is obtained when the ratio of stress on loading to the stress on unloading at the respective inflection points on a stress-strain curve in alloy deformation is at least about 2.5:1. This stent exhibits superelasticity at the living body temperature after it is released from the catheter but does not sufficiently solve the above-mentioned problem (arbitrariness in positioning) as required in PTCA.
In Patent Document 7, the present inventors have proposed a stent closely related to this invention. Specifically, proposal is made of the stent which exhibits no shape memory at the living body temperature during insertion into the living body and exhibits superelasticity after shape recovery by inflation of a balloon. In the embodiment of Patent Document 7, it is described that the stent made of a Ti—Ni alloy or a Ti—Ni—X alloy (X═Cr, V, Cu, Fe, Co, or the like) is subjected to strong deformation to thereby elevate a recovery temperature. However, problems are left in shape recovery characteristics after application of strain and adaptability to a slot shape.    Patent Document 1: U.S. Pat. No. 3,174,851    Patent Document 2: JP S58-161753 A    Patent Document 3: JP S63-171844 A    Patent Document 4: JP S63-14834 A    Patent Document 5: U.S. Pat. No. 4,770,725    Patent Document 6: JP H11-42283 A    Patent Document 7: JP H11-99207 A    Non-Patent Document 1: K. Otsuka and C. M. Wayman: Shape Memory Materials, CAMBRIDGE University Press (1998)    Non-Patent Document 2: D. Goldstein, E. Alexweiner: Nitinol-Based Fuse Arming Component, NSWXTR88-340 (1980)